Combine the flour,oats, brown sugar, pecans, cinnamonand salt in a medium bowl.
Add the buttercubes and use your fingers to completely work the butter into the flour mixture. The mixture should hold its shape when squeezed.
Sprinkle the crumble topping over the filling, squeezing some of the mixture into small clumps.
Bake until the filling is bubbly, the apples are tender and the topping is golden brown, about 1 hour.
Let sit at least 45 minutes and serve slightly warm or at room temperature.